Image quality in radiography: ISO 19232

A radiograph that looks good is not yet a usable radiograph. Whether sensitivity was sufficient to find the defects sought must be demonstrated. That is what the image quality indicator, the IQI, is for.

This article explains the types available, how to establish the required class and why a radiograph without a visible IQI wire is formally worthless.

The wire indicator

ISO 19232-1 describes the wire indicator: a row of parallel wires of increasing diameter in a plastic holder, each numbered. The indicator is placed on the component, preferably on the source side, across the weld.

A specified wire number must be visible over a defined length on the radiograph. If that wire cannot be seen, sensitivity is insufficient and the exposure is invalid. Versions in different materials exist so the indicator matches the material under examination.

The duplex wire indicator

ISO 19232-5 describes the duplex wire indicator, used to measure geometric unsharpness and, on digital images, basic spatial resolution. Two closely spaced wires merge on the image once unsharpness becomes too great.

That measurement matters most in digital radiography, where detector pixel size adds a further limit. With film, unsharpness is mainly a question of source size and distances.

Determining the required class

ISO 19232-3 gives, per material thickness and technique, which wire number must be visible. The requirement is stricter for class B than class A of ISO 17636, and stricter for single wall than for double wall exposure.

The applicable class follows from the product standard: EN 13445-5 for pressure vessels, EN 1090-2 for structural steelwork, or the project specification. Transfer that requirement literally into the NDT procedure so the operator knows which wire number to achieve. See radiographic testing.

Where reports fail

Three recurring findings when reviewing RT reports: the IQI is not visible on the image, the IQI lies on the detector side without this being stated and compensated, and the achieved wire number is not recorded.

Each of those three makes the radiograph unusable as evidence, however sharp the image looks. When reviewing a dossier that is one of the first checks DEHAAS performs. See NDT coordination.